1. Field of the Invention
The present invention relates to a lens meter for measuring optical characteristics of a lens for an eye such as a spectacle lens.
2. Description of Related Art
Conventionally, there is known a lens meter which places a lens on a measurement optical axis, projects a measurement light bundle onto the lens and measurement targets and photo-receives the measurement light bundle passed through the lens and the measurement targets by using a photo-sensor, and then obtains optical characteristics of the lens based on the photo-receiving results. This kind of lens meter previously obtains coordinates (positions) of images of the measurement targets detected by the photo-sensor in a reference state where the lens is not placed on the measurement optical axis (a lens-absent state), then obtains coordinates (positions) of images of the measurement targets detected by the photo-sensor in a measurement state where the lens is placed on the measurement optical axis (a lens-presented state), and calculates the optical characteristics of the lens based on a change in the coordinates in the measurement state with respect to the coordinates in the reference state.
Besides, in the conventional lens meter, the measurement targets are arranged limitedly within a range narrower than a photo-receiving surface of the photo-sensor. Therefore, when a portion which is not in the vicinity of an optical center of the lens such as an add section of a progressive lens or a bifocal lens is measured, there is a case where a part of the measurement target images is not detected due to generation of prism power (diopter), so that the measurement target images necessary for measurement come to short. Although there is a photo-sensor with a large photo-receiving surface, it is not easily available and is expensive.
When the measurement targets are arranged in a range wider than the photo-receiving surface of the photo-sensor in order to cope with the problems as above, some measurement target images are inevitably not detected in the reference state, and coordinates with so-called a “0D (diopter) reference” cannot be obtained for those measurement target images.